In general, a fabrication process for a semiconductor device such as a flat panel display device and a solar cell includes repetition of a deposition step of a thin film, a photolithographic step for a photoresist (PR) pattern and an etch step of the thin film for a thin film pattern. The photolithographic step includes steps of forming a PR layer, exposing the PR layer and developing the exposed PR layer to form the PR pattern. PR materials for g-line of a wavelength of 436 nm, i-line of a wavelength of 365 nm or krypton fluoride (KrF) of a wavelength of 248 nm have been used for the PR layer, and the PR pattern is formed by pattering the PR layer using a photo mask. In addition, the thin film pattern is formed by patterning the thin film using the PR pattern as an etch mask.
Recently, a method of forming a minute thin film pattern has been required for a highly integrated semiconductor device. However, a critical dimension of a thin film pattern formed by a PR pattern is restricted to about 60 nm because of limitation of the photolithographic step. Accordingly, a method of forming a minute thin film pattern having a critical dimension smaller than about 50 nm using amorphous carbon (a-C) and silicon oxynitride (SiON) as an etch mask has been suggested.
FIGS. 1A to 1D are cross-sectional views showing a method of forming a thin film pattern using amorphous carbon and silicon oxynitride according to the related art. In FIG. 1A, a thin film 20 is formed on a substrate 10 and an amorphous carbon layer 30 is formed on the thin film 20. In addition, a silicon oxynitride layer 40 and an anti-reflective layer 50 are sequentially formed on the amorphous carbon layer 30. A photoresist (PR) layer 60 is formed on the anti-reflective layer 50. Next, a photo mask M is disposed over the PR layer 60, and the PR layer 60 is exposed to light F through the photo mask M.
In FIG. 1B, a PR pattern 60a is formed by developing the PR layer 60 (of FIG. 1A). Next, after the anti-reflective layer 50 and the silicon oxynitride layer 40 are sequentially etched using the PR pattern 60a as an etch mask, the PR pattern 60a and the etched anti-reflective layer 50 are removed.
In FIG. 1C, a silicon oxynitride pattern 40a is formed on the amorphous carbon layer 30. In FIG. 1D, after the amorphous carbon layer 30 is etched using the silicon oxynitride pattern 40a as an etch mask to form an amorphous carbon pattern 30a, the silicon oxynitride pattern 40a is removed. Next, the thin film 20 is etched using the amorphous carbon pattern 30a as an etch mask to form a thin film pattern 20a. 
The amorphous carbon layer 30 may be formed by a spin-on method or a plasma enhanced chemical vapor deposition (PECVD) method. However, since an etch selection ratio between the thin film 20 and the amorphous carbon pattern 30a is relatively low, the amorphous carbon pattern 30a is required to have a thickness similar to a thickness of the thin film 20. Since an etch rate of the amorphous carbon layer 30 by a spin-on method or a PECVD method is similar to an etch rate of the thin film 20, the thin film pattern 20a may have deterioration such as a striation or a wiggling after the thin film 20 is etched using the amorphous carbon pattern 30a as an etch mask. Alternatively, the amorphous carbon pattern 30a may be eliminated before the thin film pattern 20a is formed.
To solve the above problems, a thickness of the amorphous carbon pattern 30a increases. However, when the thickness of the amorphous carbon pattern 30a increases, a resolution of the amorphous carbon pattern 30a decreases. FIG. 2 is a cross-sectional view showing deterioration of a thin film pattern according to the related art. In FIG. 2, the amorphous carbon pattern 30a may be removed more than the thin film 20 (of FIG. 1C) during the etching step for the thin film pattern 20a. In addition, the thin film pattern 20a may have a poor profile or a poor critical dimension (CD). Furthermore, increase in the thickness of the amorphous carbon pattern 30a may cause deterioration such as an aspect ratio dependent etch rate (ARDE) phenomenon, reduction in etch rate and difficulty in control of the profile.